This invention relates generally to piezoelectric transducers, and more specifically to a method for creating multiple layers of polarization, each layer being controllable in phase and depth, between two surfaces of a surface active piezoelectric transducer element.
Heretofore, nearly all of the transducer elements which must be artificially polarized to produce a piezoelectric effect have been uniformly polarized in the thickness or lateral dimension of the element. Conventional transducers of this artificially polarized category, i.e., ferroelectric ceramic materials such as barium titanate or lead zirconate--lead titanate or lead metaniobate, are polarized completely or uniformly through the element in a direction parallel with the applied polarizing potential or bias voltage.
Polarization of such ferroelectric ceramics refers to the physical condition of the crystals of the material such that its magnetic dipoles are aligned in a given direction. A material may thus be polarized in one of two different directions, i.e., the magnetic dipoles of the material may be aligned in one of two opposing directions, or the material may be unpolarized, a condition wherein the dipoles are arranged in random directions with respect to one another.
Theoretically, polarization of such a material is accomplished by applying a strong electric field to the material, and coincidentally heating it to a temperature above its Curie point. When the material is heated above its Curie temperature, it loses its ferroelectric properties, and the electric field aligns the magnetic dipoles of the material to the direction of the applied field. Practically, however, the material is heated to a temperature slightly below its Curie point, during which time a strong electric field is applied to the material. The material is thus allowed to polarize over an extended period of time. With the electric field still applied to the material, it is slowly cooled to an ambient temperature. When the external field is then removed, a remnant polarization is retained in the material and the ferroelectric ceramic now will typically respond in a manner similar to that of other natural piezoelectric materials such as Rochelle salt or ammonium phosphate crystals.
The effect which the polarized ceramic exhibits is known as the piezoelectric effect, which refers to the material's capability of mechanically deforming in response to an applied electrical signal, and conversely, of storing an electric charge in response to a mechanical or acoustical excitation. By placing electrodes on opposing surfaces of the element, this stored charge may be released in the form of an electrical current, the value of which is a function of the applied mechanical or acoustical force. The polarized ferroelectric element, which exhibits this piezoelectric characteristic, is thus a true transducer, a device capable of converting or transforming directly between various forms of energy, in this case between mechanical or acoustical energy, and electrical energy.
Conventional transducers made from such polarized ferroelectric materials, however, have several distinct limitations, one of the most important being their incapability of producing a broadband response to excitation. For instance, all piezoelectric transducers have natural resonant frequencies, wherein the transducer element vibrates in a ringing fashion after it is struck with wave energy of a certain frequency. A conventional fully polarized transducer has both a natural mechanical resonant frequency, and at least one natural electrical frequency of resonance. The period of this natural resonant frequency will be equal to twice the velocity of the wave striking the element times the thickness of the element. If the element is excited by a one-quarter wave length or greater signal with respect to its period, the transducer will tend to ring, producing an undesirable output. This ringing response of a particular transducer will inherently limit the frequency range of adequate response to mechanical or electrical signals applied to it. Thus, the effective response frequency range of a conventional piezoelectric transducer is limited to those excitation frequencies which do not cause the transducer to respond in a ringing fashion. The conventional piezoelectric transducer is thus a narrow band transducer, and often rather limited in application.
Another significant limitation of current transducer technology is the inability to produce homogeneous transducer elements which may be partially polarized in their thickness direction, such that multiple layers of different phase, area, and depth of polarization may be achieved. For instance, it may be desirable to have bimorph or multimorph transducers which are comprised of multiple layers, each layer having a particular polarization in one direction or another, or alternatively being unpolarized. FIG. 8 shows a stacking of two such elements 14 and 15, the two elements being mounted on a stationary wall 19 and oppositely polarized. The application of a voltage to this configuration will cause the device to bend in one direction or the other, depending on the polarity of the voltage. This configuration is known in the art as a bender transducer. Other configurations or additional layers may be used to accomplish other results, such as high voltage generators.
Additionally, it may be desirable to have controllable surface area polarization so that multiple transducers could be present in a single element. Thus, different area polarization configurations, as well as phase and depth configurations, within a single element have specific practical applications.
Prior art transducer technology accomplishes this multimorph transducer structure by utilizing several distinct elements, polarizing them completely in one direction or another or maintaining them unpolarized, and then combining them in discrete layers and areas, as necessary to perform the desired function. This technique, of course, results in distinct interfaces between adjoining layers, and the inherent impedance mismatch at the layer interfaces often can create significant difficulties with respect to the desired response.
Various attempts have been made to dampen the ringing effect of the transducer due to its resonant frequencies discussed above. Some of these techniques include attaching a material of closely matching acoustic impedance to the back surface of the transducer element, epoxy bound casting of piezoelectric powders in the transducer element, sputtering of piezoelectric materials, and exercising some control over the shape of the driving function of the transducer. All of these methods and techniques attempt, in one fashion or another, to dampen the natural ringing response of the transducer, either by changing the structure of the transducer element itself, or by providing some specified acoustical backing material for the element. None of these techniques, however, result in a broad band transducer element. They are all basically subject to the ringing limitations mentioned above. They each produce transducers having a slightly enhanced response in certain frequency areas albeit in exchange for other disadvantages.
U.S. Pat. to Baerwald, No. 2,659,829, discloses a technique which bears to some extent on both of the significant problems noted above, but which itself has significant disadvantages. Baerwald discloses a technique of partially polarizing a ferroelectric ceramic material in its thickness direction, resulting in a transducer which is claimed to be inherently damped, and which has a polarized layer and unpolarized layer in the thickness direction. Baerwald utilizes the steps of first completely polarizing the element in one direction by means of known techniques, and then heating a portion of one surface of the polarized material to a temperature either close to or above the Curie temperature of the material for a sufficient length of time such that the polarization of that surface layer is destroyed. The application of the heat as described in Baerwald, however, does not appear to be controlled, as there are no limits specified with respect to the quantity of heat or the length of time that the heat is applied to the surface. There is no disclosure concerning the control of the respective depths of the polarized and unpolarized layers, and hence, the response of the transducer. This results in a transducer element having unpredictably different response characteristics.
Furthermore, the transducers disclosed are not capable of being utilized as either bimorph or multimorph structures. Rather, the technique is specifically directed to producing transducers wherein a certain portion of the transducer in its thickness direction may be depolarized, for purposes of impedance matching and damping off of the back surface of the transducer. Additionally, the transducers fabricated by the method described have proven in practice to be unstable in operation, and highly susceptible to fatigue and other damage.
In view of the above, it is a general object of the present invention to provide a method of polarizing ferroelectric elements which overcome the disadvantages of the prior art.
It is another object of the present invention to provide a method of polarizing a ferroelectric material to any desired depth between an opposing pair of element surfaces.
It is another object of this invention to provide a method for polarizing a ferroelectric material so as to produce a damped piezoelectric transducer.
It is a further object of the present invention to provide a method for polarizing a ferroelectric element such that discrete layers of polarization and nonpolarization may be achieved within a single ferroelectric element.
It is a further object of the present invention to provide a method for producing a polarized transducer element wherein the response of the transducer may be significantly damped within the transducer element itself.
It is a still further object of the present invention to provide a method for polarizing ferroelectric transducer elements in which the depth, area, and phase of multiple layers of polarization within a single ferroelectric element may be controlled. Other objects, features and advantages of the invention will become apparent as the description proceeds. A method for monitoring the polarization condition of the transducers made by the present invention is more fully covered in copending application entitled "Method of Controlling the Polarization Condition of Transducers" by Norman E. Dixon and is assigned to the same assignee as the present invention.